The zoom lens on even a modest-priced camcorder is actually an optical marvel. It’s the product of years of research: it focuses itself, changes its own aperture and even changes its focal length from wide to telephoto at the push of a button. But somehow it seems the images that give us the most satisfaction are those that are just beyond its reach. We want closer, farther, bigger, smaller or just a different color. There are ways to extend the boundaries of the camcorder lens but first let’s review the basics of how the optical marvel works.
You can see how a lens forms an image if you point a magnifying glass at a brightly lit window and hold a piece of paper behind it. Move the paper back and forth to find the right distance and you will see an inverted image of the window on the paper. This is what your camcorder’s lens is doing when you shoot video: it’s focusing all of the light it gathers onto the CCD (charge coupled device), the chip which then turns the focused light into a video signal.
Now you might ask, after viewing the lens diagram in figure 1, what are all those other hunks of glass doing in my lens? The moving elements you see when you peer down the front of the lens as you zoom in and out are there to change the focal length; the others are there to correct lens aberrations. Look at the image formed by the magnifying glass again. It’s actually pretty fuzzy. This is because light rays from the outer edges of the lens don’t focus at exactly the same point as those coming through near the center. That’s called spherical aberration. Another example is chromatic aberration. We’ve all seen white light, passing through a triangular prism, spread out into a rainbow. This is because the different colors are refracted at slightly different angles. The same thing happens in a simple lens–the different colors don’t focus in exactly the same place.
There’s more to all this aberration stuff but you get the idea. Those elements inside your lens are the result of carefully designed compromises to correct all these little problems.
To understand lens speed it’s important to distinguish between the quantity of light and the intensity of light. It takes a specific quantity of light to enable the CCD in your camcorder to produce a good quality signal. You can provide the right quantity of light in a short time with a bright light or in a longer time with a dim light. Since your camcorder must generate 60 fields per second, it must usually get the job done in under 1/60th of a second.
If you look at the barrel of a professional video lens (not included on consumer models), you will see numbers like 4, 5.6, 8, etc. Called f-numbers or f-stops, they represent the focal length of the lens divided by the diameter of the iris opening. At f/4, the opening is 1/4 of the focal length, while at f/8, the opening is only 1/8 of the focal length. This quotient is a measure of the intensity of the light reaching the camcorder’s CCD. Small numbers indicate more light intensity (through a larger hole) while larger numbers admit a lower light intensity and require a longer exposure (or a lower shutter speed setting). The speed of the lens refers to the lowest f-number attainable by a lens. For example, a lens whose maximum aperture is f/2 is called an f/2 lens.
Accessorize Your Lens
Now that you’re up to speed on lens basics, let’s look at some of the ways to extend the capabilities of your lens. Before we begin, a word of caution about lens diameters. If you look at the front of your lens, you’re likely to see a number that corresponds to the diameter of the threads on the outside of your lens. Before you purchase any lens accessories, be sure you know the diameter of your lens; if you buy something with the wrong diameter, it won’t fit your lens without an adapter. Some lens accessories are the clip-on variety, which means they’ll fit a wide variety of lenses-even those without threads.
One of the easier ways to modify your lens is through the use of closeup (or diopter) lenses. Simply add one or two to the front of your lens to enable it to focus closer than it otherwise could. You will see these lenses labeled +1 diopter, +3 diopter, etc.
Each diopter number is the reciprocal of the focal length in meters, i.e., one divided by the focal length. For example, a 1-meter focal length is a +1 diopter lens while a 0.5 meter (500 millimeter) focal length is a +2 diopter lens (1/0.5).
When you combine two or more diopter lenses, calculate the combined effect by adding the reciprocals of their focal lengths. Therefore, if you label the lens with its reciprocal focal length, you can simply add the diopter numbers to get the combined effect. A +1 diopter lens together with a +2 diopter lens gives the effect of a +3 diopter lens.
Before you fantasize too much about stacking up lens after lens, you should know that two are the most you can use. Remember all those carefully designed elements to correct lens aberrations? When you start throwing in extra hunks of glass, you start fouling up all that careful design work. Moreover, as the stack of lenses grows the more likely you are to get vignetting (black circles cutting off the corners of your image at wide-angle settings).
Century Precision Optics makes a set of high-quality diopter lenses that fit most consumer camcorders. Available in +1, +2, +4 and +7 magnifications, these diopters sell for $250 each.
Wide-Angle and Telephoto Converters
One of the things most video enthusiasts strive for is to extend their lens’s zoom range to longer or shorter focal lengths. That wildlife seems always to be too far away or the wall is always too close behind us to get the whole room in the frame. Fortunately, there is an abundance of wide-angle and telephoto attachments to solve these problems.
Wide-angle adapters simply slip over your lens and increase its angle of view. Adapters are available in a variety of strengths from merely extra wide to super wide or fisheye lenses. Examples of wide angle adapters are the Century Precision Optics 0.6x adapter ($295), the Raynox 0.5x adapter ($60) and the Adorama 0.5x adapter ($43).
Also available for your shooting pleasure are a number of telephoto adapters. As you would expect, these devices extend your lens in the long focal length direction and pull those distant objects in close. Look for the Kenko 5x ($100), the Sony 2x ($50), the Raynox 3x ($125) or the Adorama 3.5x ($90).
One thing that is often overlooked in the use of telephoto adapters is that they usually cost you one or two stops in lens speed. Wide-angle adapters may also cause a small reduction in speed. Be alert for this in low-light situations.
A neat trick for the budget-conscious videographer is a two-in-one device that offers both wide-angle and telephoto supplements. By simply flipping the lens end for end you can go from 2x telephoto to 0.5x wide angle. A number of companies make these adapters; Adorama sells one for $55.
All of these lens accessories come in a wide variety of prices and qualities. The least expensive devices may have plastic lenses while the higher quality lenses will be glass but priced steeply.
Filters and Other Accessories
There is a truly bewildering array of filters available–from polarizing filters that can reduce the reflections coming off a shiny surface to every variety of colored filter you can imagine.
One type of filter worth mentioning is the 2A haze or skylight filter. It absorbs UV light and therefore reduces haze in distant outdoor shots, giving everything a slightly warmer tone. Keeping one on your camcorder anytime you are shooting in a hostile environment will protect your expensive lens.
Another common filter type is the neutral density filter. Sold in varying strengths (labeled ND2, ND4, etc.), these filters do nothing more than reduce the amount of light coming into the camera. Higher ND numbers mean less light is admitted. The most common reason for doing this? So that you can open up the iris of the camcorder and reduce the depth of field-the range of the picture that appears in sharp focus–while still maintaining proper exposure levels.
Canon sells a set of accessory filters (model FS-46U) that includes a 2A skylight filter, an ND2 neutral density filter and a polarizing filter for $45.
Some uncommon types of lens accessories include the Adorama Spy Scope ($30), which allows you to shoot around a corner at a 90 degree angle, and the Raynox Video Microexplorer ($200), which includes a light table and three microscopic lenses (35x, 70x and 140x) for shooting very small objects.
As with all gadgets, it’s always wise to examine your needs first. What sort of footage are you most interested in accumulating? Once you’ve answered this question, go after the best quality lens accessories you can afford.
Bob Stephenson has been a photographer for 50 years and a videographer since 1995.
Lens Accessory Manufacturers
This list is only a sampling. It is not meant to be comprehensive.
42 West 18th Street
New York, NY 10011
One Canon Plaza
Lake Success, NY 11042
Century Precision Optics
11049 Magnolia Blvd.
North Hollywood, CA 91601
THK Photo Products, Inc.
1512 Kona Drive
Compton, CA 90220
150 20th Street
Brooklyn, NY 11232
Sony Consumer Electronics
One Sony Drive
Park Ridge, NJ 07656
Tiffen Manufacturing Corp
90 Oser Avenue
Hauppage, NY 11788